Frost heaving (or
frost heave) is
the process by which the freezing of water-
saturated soil causes the
deformation and upward
thrust of the ground surface. This process can damage
plant roots through breaking or
desiccation, cause cracks in
pavement, and damage the
foundation of buildings, even
below the
frost line. Moist, fine-grained
soil at certain
temperatures is most
susceptible to frost heaving.
Frost creep, an effect of frost heave, involves a
freeze-thaw action allowing
mass
movement down
slope. The
soil or
sediment is frozen and
in the process moved upward
perpendicular to the slope. When
thaw occurs the sediment moves downwards thus mass
movement occurs.
Mechanisms
Molar volume expansion
The earliest known documentation of frost heaving came in the
1600s. The
molar volume of water
expands by about 9% as it transforms from water to ice at its bulk
freezing point. Originally, frost
heaving was thought to occur due simply to the freezing of
water that was present in the soil prior to the onset
of subzero temperatures, and which froze in place without moving.
If this were the sole source of expansion, 9% would be the maximum
expansion possible, and even then only if the ice were rigidly
constrained laterally in the soil so that the entire volume
expansion had to be taken up vertically. However, the vertical
displacement of soil in frost heaving can be significantly greater
than that due to molar volume expansion.
Ice is
unusual in that there is an increase in molar volume when freezing.
Most compounds show contraction on transforming from liquid to
solid. Classic expermients by Taber first demonstrated a flow of
liquid water towards a cold front, and that liquids such as
benzene, which contracts when it freezes,
also produces frost heave, thereby eliminating molar volume changes
as the only mechanism for vertical displacement. These experiments
also demonstrated the generation of
ice
lenses inside columns of soil that were frozen by cooling the
upper surface only, thereby establishing a
temperature gradient. Therefore the
molar volume expansion of water cannot be the sole, and may not be
a major, contributor to frost heave.
Liquid water source, transport, and existence below the bulk
freezing point
As the heaving may be greater than that possible due to the 9%
expansion of water on freezing, this requires more water to flow
into the freezing region, which in turn requires a source of
liquid water and a means of transport. During
frost heave, one or more soil-free ice lenses grow, and their
growth displaces the soil above them. One source of water is from
water at depth, where the temperature is above the bulk freezing
point. However, at the ice lens, the temperature is obviously at or
below the bulk freezing point. However, this does not shut off the
water supply, as liquid water can exist below its bulk freezing
point. One effect that allows for liquid water to exist below the
bulk freezing point is the
Gibbs-Thomson effect of confinement of
liquids in pores. Very fine pores have a very high
curvature, and this results in the liquid phase
being the
thermodynamically
stable phase in such media at
temperatures sometimes several tens of degrees below the bulk
freezing point. Flow of liquid water through fine pores would be
one mechanism to supply growing ice lenses in soils. Another effect
is the preservation of a few atomic layers of liquid water on the
surface of ice, and between ice and soil particles. This unfrozen
layer of water is also known as
premelted
water and has been known to exist since the nineteenth century. Ice
premelts against its own
vapour, and in
contact with
silica.
Thermal regelation
The same intermolecular forces that cause premelting at surfaces
have been shown to cause heaving. If ice surrounds a fine soil
particle against which it premelts, the soil particle will be
displaced in the direction of the thermal gradient due to melting
and refreezing of the thin film of water that surrounds the
particle. The thickness of such a film is temperature dependent and
is thinner on the colder side of the particle. Water has a lower
free energy when in bulk ice than when
in the supercooled liquid state. Therefore, there is a continuous
replenishment of water on the cold side, by flow of water from the
warm side to the cold side, and continuous melting to re-establish
the thicker film on the warm side. The particle is forced towards
the warm direction. This process is called
thermal
regelation The ice repels foreign particles, and a 10
nanometer film of unfrozen water around each
particle can lift a
micron-sized particle by
10 microns/day in a thermal gradient of as low as 1
Km
-1. Therefore the ice lens can purge itself of any
particles that are entrained, and tends to reject them at its
interface in the first place. The ice lens can both lift the soil
above it and itself, by pushing particles downwards towards its
lower (warmer) interface, which clearly must remain at or below the
bulk freezing temperature. If the air temperature is below freezing
but relatively stable, the
heat of
fusion from the water that freezes can cause the temperature
gradient in the soil to remain constant.
As the liquid water freezes onto the ice lens, soils draw in
further liquid water from the network of unfrozen films that exist
on the scale of a few nm in the soils around them. In doing so the
free energy of the whole system is lowered.
Susceptible soil types
Frost heave relies on soils in which there is a supply of liquid
water to feed growing ice lenses, established in a thermal
gradient, that are capable of displacing the soil perpendicular to
that gradient. This requires:
- freezing temperatures
- a supply of water
- a soil that has:
- the ability to conduct water
- a high affinity for water
- saturation (i.e. the pore spaces are filled with water)
Silty and loamy
soil types are
susceptible to frost heaving. The affinity of a soil for water is
generally related to the surface area of the particles that it is
composed of.
Clays have a high ratio of surface
area to volume and have a high affinity for water. Larger particles
like
sand have a lower ratio of surface area to
volume and therefore a low affinity for water.
Conversely, the hydraulic conductivity of a soil is related to the
pore size. Soils composed of very small particles like clay have
small pores and therefore low hydraulic conductivity. Soils
composed of larger particles like sand have larger pores and a
higher hydraulic conductivity.
The offsetting nature of these two requirements mean that clayey
and sandy soils are less conducive to frost heaving than
silt, which has a moderate pore size and moisture
affinity.
Frost creep: Soil locomotion due to frost heave
Frost creep, an effect of
frost heave, involves a
freeze-thaw action allowing mass movement down
slope. The
soil or
sediment is frozen and in the process moved upward
perpendicular to the slope. When
thaw occurs the sediment moves downwards thus
mass movement, or locomotion, occurs.
Structures created by frost heaving
In Arctic
regions, frost heaving for hundreds of years can create structures,
known as pingos
, as high as
60 metres. Frost heaving is also responsible for creating
stones in unique shapes such as circles, polygons and stripes.
A notable
example is the remarkably circular stones of the islands of
Spitsbergen
.
Polygonal forms caused by frost heave have been observed in
near-polar regions of Mars by the high-resolution
HiRISE camera on the
Mars Reconnaissance Orbiter. In
May 2008 the
Mars Phoenix lander
touched down on such a polygonal frost-heave landscape and quickly
discovered ice a few centimetres below the surface.
For further information, see:
patterned
ground.
See also
- Frost law
- Lithalsa
- Pingo, also called
hydrolaccolith, a mound of earth-covered
ice found in the Arctic and subarctic area that can reach up to 70
m in height and up to 600 m in diameter.
- Palsa, a low oval elevation in areas with
permafrost, frequently peat bogs, where a perennial ice lens has developed
within the soil.
References
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